Turbine blades generally include an airfoil and a tip shroud attached thereto. The tip shroud, which attaches to the outer edge of the airfoil, provides a surface area that runs substantially perpendicular to the airfoil surface. The surface area of the tip shroud helps to hold the turbine exhaust gases on the airfoil (i.e., does not allow the exhaust gases to slide over the end of the airfoil blade) so that a greater percentage of energy from the turbine exhaust gases may be converted into mechanical energy by the turbine. Thusly, tip shrouds improve the performance of gas turbine engines. The preferred tip shroud design calls for a large tip shroud surface area such that the entire outer surface of the airfoil of the turbine blade is covered.
During turbine operation, a tip shroud generally interacts with the tip shrouds of adjacent turbine blades. That is, because of the alignment of installed turbine blade and the preferred tip shroud design, a tip shroud generally makes contact with the tip shrouds on each side of it, i.e., the adjacent tip shroud on its leading edge and trailing edge. The contact that is made between the tip shrouds of adjacent turbine blades also may help to hold the turbine exhaust gases on the airfoil (i.e., prevent significant leakage between the tip shrouds) such that turbine performance is enhanced. However, given the rotational velocity and vibration of the turbine in operation and the non-permanent nature of the joint made between adjacent tip shrouds, the physical and mechanical stresses associated with the contact between adjacent tip shrouds are extreme.
In addition, turbine blades of industrial gas turbines and aircraft engines operate in a high temperature environment. In general, the temperatures in the turbine where the turbine blades operate are between 600 and 1500° C. Further, the rapidity and frequency of changes in turbine operating temperatures exacerbate the thermal stresses applied to hot-path components. As a result, the thermal stresses on turbine blades and the tip shrouds attached thereto are extreme.
Turbine blades and tip shrouds attached to them generally are made of nickel-based super alloys, cobalt-based super alloys, iron-based alloys or similar materials. While these materials have proven cost-efficient and effective for most necessary functions, given the extreme mechanical and thermal stresses, the connective area between adjacent tip shrouds (i.e., where a tip shroud contacts each of the tip shrouds adjacent to it) tend to wear prematurely. Other harder/more durable materials are more effective at resisting the kind of wear that occurs at the contact areas between adjacent tip shrouds.
Conventional methods and systems have been unsuccessful at preventing this wear in an effective manner. For example, flame spray coatings; have been tried. However, such coatings have proven to be too thin to provide any long-lasting protection. Specialized welding, which generally constitutes “weld build-up,” in the contact area also has been tried. However, specialized welding also has shown to provide little protection. Further, weld build-up introduces further heat related stresses to the contact area, when operational stresses in this area already are extreme.
As a result, premature wear at the contact point between adjacent tip shrouds continues to result in system inefficiencies. For example, premature wear may cause: 1) increased repair downtime to the turbine unit; 2) replacement of otherwise healthy tip shrouds due to the premature wear in the area of contact; and 3) related increases in labor and part expenses. Thus, there is a need for improved systems for protecting against premature wear between adjacent tip shrouds.